Preheat oven to 400 degrees.
Evenly coat a casserole baking dish or 10" cast iron skillet with 2 tablespoons of melted butter.
Prepare elbow macaroni according to instructions on package. Cook until al dente (firm). Mushy macaroni noodles not suggested for this recipe. Drain cooked noodles and place in buttered casserole dish or 10" cast iron skillet. Set aside.
In a medium bowl, combine buttermilk, beaten egg (if using), sour cream, salt (as preferred), coarse ground black pepper (as preferred), garlic powder, onion powder, paprika (if using). Stir until evenly combined.
Pour mixture in center of baking dish. Stir until evenly combined.
Top with 1/2 of each block of cheese (shredded or thinly sliced). Cover with foil.
Reduce oven temperature to 375. Bake 8 to 10 minutes OR until cheese is melted.
Remove from oven. Stir together melted cheese and noodles. Top with remaining 1/2 blocks of cheese (shredded or thinly sliced).
Sprinkled crushed French fried onions over the top of each serving (optional).
Return to HOT oven. Cook another 8 to 10 minutes OR until cheese is bubbly.
If desired, place under boiler to brown (optional).
Serve hot.